This invention relates to semiconductor devices, and more specifically relates to a solderable front metal contact for structures employing an MOS type of gate structure.
Many types of MOS devices are well known in which the conduction path through the device is vertical so that one surface, commonly known as the front surface, contains a source or cathode electrode while the rear surface contains a drain or anode electrode. The front or top surface will frequently contain the device gate or control electrode as well. Such MOS devices are power MOSFET devices like the HEXFET MOSFET manufactured and sold by the International Rectifier Corporation, the assignee of the present invention, and the insulated gate bipolar transistor device, also manufactured and sold by the assignee of the present invention. These devices are sometimes referred to hereinafter as "MOSGATE" structures.
The back metal of such devices is a material which can be soldered to a heat sink to remove heat from the chip. However, the front metal used for the source or cathode electrode is traditionally aluminum which is not solderable.
Aluminum front metal is conventionally used for such products because it makes a relatively low-resistance contact to the bare silicon of the chip and lends itself to photolithography processing with considerable accuracy. However, the thin aluminum conventionally used as a source contact and source contact pad in a power MOSFET or as the cathode contact and contact pad in an IGBT-type device tends to have relatively high lateral resistance. The aluminum front contact, therefore, does not provide uniform current distribution in power devices which carry relatively high currents. Furthermore, the aluminum pad does not permit the connection of a relatively massive heat sink conductor to the front surface of the chip to assist in dissipation of heat generated within the chip during its operation. Thus, most of the heat in such devices is removed through the back contact. If it were possible to employ a massive front contact, more heat could be removed from the chip and the same chip can be operated at higher current.
Another disadvantage of the relatively thin aluminum front contact is that the source lead is connected to the pad by a wire-bonding process. During the wire-bonding process, the pressures involved can damage body cells of the device disposed beneath the pad. Thus cells are either not formed under the pad region, significantly reducing the active area, or one runs the risk of damage to the cells during wire bonding.
Known semiconductor devices such as bipolar power transistors and power thyristors have employed solderable contacts on both front and back surfaces so that heat can be removed from such chips from both surfaces. However, in the manufacture of bipolar transistors or other similar high power semiconductor devices having both front and rear solderable contacts, the device surfaces do not require thin aluminum electrodes for their processing and do not contain the complex and fragile junctions of a multicellular nature as required for a MOSGATE-type device.